UV-activated dielectric layer

ABSTRACT

A dielectric layer on a semiconductor substrate is made porous by radiation with UV light. The dielectric material contains a photosensitive moiety that absorbs UV radiation and dissociates from the dielectric material. The UV-activated material then may be diffused to create pores in the dielectric layer, and to provide a dielectric layer having a low dielectric constant.

FIELD OF THE INVENTION

This invention relates generally to semiconductor devices having one ormore dielectric insulating layers with a low dielectric constant, andmethods for making porous dielectric layers in such devices.

BACKGROUND

Semiconductor devices include metal layers that are insulated from eachother by dielectric layers. As device features shrink, reducing thedistance between the metal lines on each layer, capacitance increases.The parasitic capacitance may contribute to effects such as RC delay,power dissipation, and capacitively coupled signals, also known ascross-talk. To address this problem, insulating materials that haverelatively low dielectric constants (referred to as low-k dielectrics)are being used in place of silicon dioxide (and other materials thathave relatively high dielectric constants) to form the dielectric layerthat separates the metal lines.

Attempts have been made to lower the dielectric constant by increasingthe porosity of dielectric materials. For example, some porousdielectric materials use thermally activated porogens. When heat isapplied, the porogen may decompose and/or volatize, leaving pores in thedielectric material. Lower dielectric constants are possible because thepores are voids having dielectric constants near one (1).

Pores in dielectric materials typically have been generated by thermalprocessing. For example, one such dielectric material is asilsesquioxane matrix containing porogens, which may be spun on to thesubstrate. Heating the material causes the porogen to decompose andvaporize, leaving pores in the dielectric layer, thereby decreasing thedielectric constant of that layer.

However, heating during other process steps can cause a porogen todecompose prematurely. For example, other process steps that requirethermal input (such as photoresist bake and ashing steps or subsequentdeposition steps for dielectric etch stop or metallic layers), maythermally decompose the porogen, resulting in various problems.Premature thermal decomposition of porogens, for example, can result inrough sidewall surfaces of metal interconnects. Additionally, someporogens, such as those based on poly(ethylene oxide) or similarmaterials, may be lost prematurely due to resist cleans that can attackor solubilize the porogen.

A porous dielectric material is needed that will not decomposeprematurely during process steps that occur at high temperatures. Amethod of providing a porous dielectric material having a low dielectricconstant is needed. A dielectric material and method is needed that willnot be susceptible to premature decomposition of porogens, for use informing dual damascene and similar interconnects in increasingly smallersemiconductor device geometries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a method of forming a porous dielectriclayer according to one embodiment.

FIGS. 2A-2B provide a schematic representation of a porous dielectriclayer formed on a semiconductor substrate in one embodiment.

DETAILED DESCRIPTION

In one embodiment, as shown in FIG. 1, a dielectric layer is formed on asubstrate, and pores are formed in the dielectric layer by applicationof ultraviolet (UV) light energy.

In block 101, a dielectric material is applied to a substrate byspin-coating, evaporative deposition, or physical vapor deposition. Thedielectric material may be a variety of materials ranging from a spin-onglass to organic materials such as paralyene. The dielectric layer maybe applied over a conductive metal layer. A barrier layer also may bepositioned between the dielectric layer and metal layer. Optionally, anon-hermetic capping layer may be formed over the dielectric layer.

The dielectric material includes one or more photosensitive moieties. Inone embodiment, the dielectric material provides a “backbone” onto whichphotosensitive moieties may be linked and functionalized.

In one embodiment, the photosensitive moiety may be the absorbent moiety(hereafter synonymously referred to as “antenna”) from a photoacidgenerator (PAG) linked to a dielectric “backbone.” Functional groupssuch as acid groups attached to the dielectric material facilitateattachment of photosensitive moieties. For example, the photosensitivemoiety may be linked to the dielectric “backbone” with a carboxylate,phnenoxy group, or other similar functionality.

Photoacid generators are salts which have an antenna functionality. Byincorporating the light absorbing moiety or antenna as a side grouplinked to the dielectric material, the PAG antenna moiety may be used asa photoactive, “backbone”-bound porogen.

Thus, the dielectric “backbone” may be functionalized with an antennafunctional group. For example a polymer dielectric “backbone” may besynthesized or functionalized to have carboxylic acid moieties that aresubsequently functionalized to form a diphenylpropylsulfonium salt.Other common photosensitive antenna moieties that may be used inembodiments of the invention include triphenyl sulfonium, alkylphenylsuulfonium, diphenyl iodonium, phenylpropyl iodonium, alkylphenyliodonium, diphenyl p-alkoxyphenyl, diphenyl p-alkylphenyl, anddibutylnaphthysulfonium salts.

Optionally, an anti-reflective and/or light absorbing layer may beapplied over the dielectric layer. This may prevent premature activationor decomposition of the PAG during lithographic patterning of thedielectric, if further processing is required before generating pores inthe dielectric material. Such further processing may include etchingtrenches or vias, depositing dielectric or metal layers, plating metallayers, chemical mechanical polish, or other processes as is apparent tothose skilled in the art.

In block 102, ultraviolet (UV) light may be used to control thegeneration of pores in the dielectric material. Irradiating thedielectric material with UV light causes the photosensitive moieties tocleave from the “backbone” and become volatile and/or become lowmolecular weight species. When exposed to UV light, the antenna absorbsthe radiation and then dissociates from the “backbone”, generating anacid (by donating a hydrogen atom through beta hydride elimination orsimilar mechanisms, or abstracting a hydrogen atom from the surroundingmedia) and several other lower energy decomposition products, such asbenzene and phenyl sulfide.

For example, a diphenylpropylsulfonium antenna moiety may dissociate andgenerate an acid (on the dielectric backbone) along with otherdecomposition products such as diphenyl sulfide and propene. Otherdecomposition products also may be created upon exposure to UVradiation, in addition to those based on β-hydride elimination. Forexample, triphenylsulfonium is an antenna moiety that dissociates intodiphenyl sulfide and benzene or a benzene derivative.

Thus, UV radiation may be used to switch the dielectric material from amechanically strong material with a relatively high dielectric constantto a mechanically weaker material with a low dielectric constant.Relatively economical flood-exposure tools may be used to provide UVlight and activate the decomposition.

In one embodiment of the invention, as shown in block 103, the substratethen may be heated to cause the low molecular weight decompositionproducts to diffuse. Optionally, diffusion may occur through anon-hermetic porous capping layer. Upon heating, the decompositionproducts such as diphenyl sulfide and propene volatilize, leaving voidvolumes in the locations previously occupied by the antenna. As aresult, as photosensitive moieties in the dielectric material decomposeand volatilize, pores are formed in the dielectric layer.

The choice of the photosensitive moiety (the “antenna”) may be based onthe volume occupied by the moiety which determines the size of theremaining void (after application of UV light and decomposition), andthe concentration of the moiety relative to the backbone, whichdetermines the void volume after decomposition.

Additionally, in one embodiment, a plasma such as H₂ may be used tomodify one or more remaining functional groups in the dielectric layer.For example, H₂ plasma may be used to reduce a carboxylate group to analkyl group or other functional groups having lower polarization, andthus, lower contribution to the overall dielectric constant of theremaining porous dielectric.

FIGS. 2A-2B illustrate cross-sections of a device that may be made usinga method according to one embodiment of the invention. In thisembodiment, UV light is provided to activate the porogen afterinterconnects are formed in or through the dielectric layer. However, inother embodiments, application of UV light may occur before theinterconnects are formed.

The device shown in FIGS. 2A and 2B includes a substrate 201 upon whichis formed a conductive layer 202. Conductive layer 202 is covered bybarrier layer 203, which in turn is covered by dielectric layer 204containing UV-activated pore-generating material.

Substrate 201 on which the dielectric may be deposited may be anysurface, generated when making a semiconductor device, upon which aninsulating layer may be formed. The substrate may include, for example,active and passive devices that are formed on a silicon wafer such astransistors, capacitors, resistors, diffused junctions, gate electrodes,local interconnects, etc. The substrate also may include insulatingmaterials (e.g., silicon dioxide, either undoped or doped withphosphorus (PSG) or boron and phosphorus (BPSG); silicon nitride;silicon oxynitride; silicon carbide; carbon doped oxide; or a polymer)that separate such active and passive devices from conductive layersthat are formed on top of them, and may include various types ofconductive layers.

Conductive layer 202 may be made from materials conventionally used toform conductive layers for integrated circuits. For example, conductivelayer 202 may be made from copper, a copper alloy, aluminum or analuminum alloy, such as an aluminum/copper alloy. Alternatively,conductive layer 202 may be made from doped polysilicon or a silicide,e.g., a silicide comprising tungsten, titanium, nickel or cobalt.

Conductive layer 202 may include a number of separate layers. Forexample, conductive layer 202 may comprise a primary conductor made froman aluminum/copper alloy that is sandwiched between a relatively thintitanium layer located below it and a titanium, titanium nitride doublelayer located above it. Alternatively, conductive layer 202 may comprisea copper layer formed on underlying barrier and seed layers.

Although a few examples of the types of materials that may formconductive layer 202 have been identified here, conductive layer 202 maybe formed from various other materials that can serve to conductelectricity within an integrated circuit. Although copper is preferred,the use of any other conducting material, which may be used to make anintegrated circuit, falls within the spirit and scope of the presentinvention.

Barrier layer 203 covers conductive layer 202. Barrier layer 203 mayinhibit diffusion into dielectric layer 204 of copper or other elementsthat may be included in conductive layer 202. In addition, barrier layer203 may perform an etch stop function—a function which may beparticularly desirable if an opening etched through the overlyingdielectric layer is unlanded. Barrier layer 203 preferably comprisessilicon nitride or silicon carbide, but may be made of other materialsthat can inhibit diffusion from conductive layer 202 into dielectriclayer 204 and provide high selectivity to etch chemistry used to etch alayer, or layers, formed on top of barrier layer 203. Other materialsthat may provide such properties include titanium nitride andoxynitride.

Barrier layer 203 should be thick enough to perform such functions, butnot so thick that it adversely impacts the overall dielectriccharacteristics resulting from the combination of barrier layer 203 anddielectric layer 204. To balance these factors, the thickness of barrierlayer 203 preferably should be less than about 10% of the thickness ofdielectric layer 204.

The dielectric layer may be a variety of materials ranging from aspin-on glass to organic materials such as paralyene. The dielectricmaterial forming the layer provides a “backbone” onto which one or morephotosensitive moieties may be linked and functionalized.

Metal interconnect 207 may be formed in an opening in the dielectriclayer. The metal interconnect may be formed by patterning openings inthe dielectric layer using dual damascene, single damascene, or similarprocesses. The openings may be trenches and/or vias patterned usingphotolithography processes, with dielectric material removed by etching.An anti-reflective or light absorbing layer may be applied overspecified areas of the dielectric layer during the lithography processto prevent premature decomposition of the UV light-activated porogens.Forming the metal interconnect also may include related chemicalmechanical polishing (CMP) or planarizing steps.

As shown in FIG. 2A, UV light source 211 directs or radiates UV light210 onto the dielectric layer. The UV light source may, for example,flood exposed portions on the surface of the substrate with UV light.

FIG. 2B shows the resulting structure that includes dielectric materialhaving pores 208. The material may be at least 50% porous, resulting ina low dielectric constant. The metal interconnects adjacent thedielectric layer also may have smooth sidewalls.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method comprising: forming a dielectric layer on a substrate, thedielectric layer including a photosensitive moiety linked to adielectric material; and radiating ultraviolet light on the dielectriclayer to unlink the photosensitive moiety from the dielectric materialto generate an acid and to form a pore in the dielectric layer.
 2. Themethod of claim 1 further comprising forming a metal interconnect in anopening in the dielectric layer.
 3. The method of claim 1 whereinradiating ultraviolet light on the dielectric layer to unlink thephotosensitive moiety generates an acid.
 4. The method of claim 1further comprising heating the dielectric layer after radiatingultraviolet light on the dielectric layer.
 5. The method of claim 1further comprising forming a porous capping layer over the dielectriclayer.
 6. The method of claim 1 further comprising forming ananti-reflective layer over the dielectric layer before radiatingultraviolet light on the dielectric layer.
 7. A method comprising:directing an ultraviolet light on a dielectric material to dissociate alight-absorbing component from the dielectric material, the dissociationgenerating a volatile material; and heating the dielectric material toremove the volatile material therefrom, the removal of the volatilematerial forming pores in the dielectric material.
 8. The method ofclaim 7 further comprising forming an opening in the dielectric materialfor a metal interconnect.
 9. The method of claim 8 wherein forming anopening in the dielectric material for a metal interconnect comprises adual damascene process.
 10. The method of claim 7 wherein thelight-absorbing component comprises a photosensitive portion of aphotoacid generator.
 11. The method of claim 7 wherein the dielectricmaterial functionality after dissociation comprises at least an acid.12. A method comprising: forming a dielectric layer on a substrate, thedielectric layer including a photosensitive moiety including a photoacidgenerator; and radiating ultraviolet light on the dielectric layer torelease the photosensitive moiety from the dielectric layer and form apore in the dielectric layer.
 13. The method of claim 12 furthercomprising forming a metal interconnect in an opening in the dielectriclayer.
 14. The method of claim 12 further comprising heating thedielectric layer after radiating ultraviolet light on the dielectriclayer.
 15. The method of claim 12 further comprising forming a porouscapping layer over the dielectric layer.
 16. The method of claim 12further comprising forming an anti-reflective layer over the dielectriclayer before radiating ultraviolet light on the dielectric layer.